Device comprising nanowires

ABSTRACT

A nanowire forming method, including the forming of a DNA origami having through openings, and the forming in the through openings of portions forming all or part of the nanowires.

The present patent application claims the priority benefit of Frenchpatent application FR19/13617 which is herein incorporated by reference.

TECHNICAL BACKGROUND

The present disclosure generally concerns nanostructures, in particularnanowires, and electronic devices using nanowires.

PRIOR ART

Nanowires are defined by elongated structures having in their transversedirections nanometer-range dimensions, that is, dimensions smaller thanone micrometer, preferably smaller than 500 nm. Nanowires are used inparticular in sensors measuring physical quantities, such as pressure orstress, causing the deformation of the nanowires. In particular, thenanowires are used in pressure sensors, gas sensors, piezoelectricgenerators, etc. The smaller the dimensions of nanowires and the morenumerous they are, the higher the resolution of the sensor and itssensitivity may be.

SUMMARY OF THE INVENTION

There is a need for nanowire manufacturing methods having transversedimensions smaller than those of nanowires obtained by usual methods.

There is a need for nanowire manufacturing methods enabling to obtain anumber of nanowires per surface area unit higher than that obtained byusual methods.

There is a need for manufacturing methods enabling to obtain nanowiresmore regularly distributed on a surface, in other words periodically,with respect to nanowires obtained by usual methods.

An embodiment overcomes all or part of the disadvantages of knownnanowire forming methods.

According to a first aspect, an embodiment provides a nanowire formingmethod, comprising the forming on a metal region of a layer havingthrough openings, and the forming in the through openings of portionsdeposited in a chemical bath, forming all or part of the nanowires andextending from the metal region.

According to an embodiment, the chemical bath for forming said portionscomprises, in solution:

-   -   a first compound defining a source of metal cations; and    -   a second compound comprising a source of hydroxide, sulfide, or        selenide ions,    -   the concentrations of the first and second compounds and/or        their ratio being lower than a concentration threshold of said        chemical bath, said threshold being such that, when the        concentrations are lower than said threshold, a growth of said        portions parallel to said layer is favored over a growth of said        portions in a thickness direction of said layer, said threshold        being preferably in the order of 10 mM; and/or    -   the chemical bath comprising one or a plurality of additives        adapted to favoring a growth of said portions parallel to said        layer over a growth of said portions in the thickness direction        of said layer, preferably citrate or chloride ions; and/or    -   the first compound being in a superstoichiometric concentration        with respect to the second compound.

According to an embodiment:

-   -   said metal cations are cations of at least one metal from the        group formed of Zn, Cd, Ni, Ag, and Cu; and/or    -   the first compound comprises at least one component from the        group formed of nitrates, acetates, chlorides, and sulfates;        and/or    -   the second compound comprises at least one component from the        group formed of HTMA, of ammonia, of sodium hydroxide, or        thiourea, of selenourea, and of sodium selenite.

According to an embodiment, said portions form first portions of thenanowires, the method comprising the chemical bath deposition of secondportions of the nanowires extending from the first portions.

According to an embodiment, the composition of the chemical bath isdifferent for the forming of the first and second portions.

According to an embodiment, the chemical bath for forming the secondportions comprises, in solution:

-   -   a first compound defining a source of metal cations; and    -   a second compound comprising a source of hydroxide, sulfide, or        selenide ions,    -   the concentrations of the first and second compounds and/or        their ratio being greater than a concentration threshold of the        chemical bath for forming the second portions, the concentration        threshold of the chemical bath for forming the second portions        being such that, when the concentrations are greater than this        threshold, a growth of the second portions orthogonally to said        layer is favored over a growth of the second portions parallel        to said layer, the concentration threshold of the chemical bath        for forming the second portions being preferably in the order of        20 mM; and/or    -   the chemical bath for forming the second portions comprising one        or a plurality of additives adapted to favoring a growth of the        second portions orthogonally to said layer over a growth of the        second portions parallel to said layer, preferably polyethylene        imide or ethylene diamine; and/or    -   the first compound being in a substoichiometric concentration        with respect to the second compound.

According to an embodiment:

-   -   said metal cations of the chemical bath for forming the second        portions are cations of at least one metal from the group formed        of Zn, Cd, Ni, Ag, and Cu; and/or    -   the first compound of the chemical bath for forming the second        portions comprises at least one component from the group formed        of nitrates, acetates, chlorides, and sulfates; and/or    -   the second compound of the chemical bath for forming the second        portions comprises at least one component from the group formed        of HTMA, of ammonia, of sodium hydroxide, of thiourea, of        selenouera, and of sodium selenite.

According to an embodiment, the method comprises the forming, at an endof the nanowires opposite to said metal region, of anelectrically-conductive region in contact with the nanowires.

According to an embodiment, the method comprises the forming of apolymer matrix between the nanowires.

According to an embodiment, the method comprises the removal of saidlayer.

According to an embodiment, the metal region comprises at least one ofthe materials of the group formed of gold, of nickel, of copper, ofpalladium, and of platinum.

According to an embodiment, the metal region has a thickness greaterthan 100 nm.

According to an embodiment, said layer is obtained by lithography from alayer comprising a block copolymer or from a layer sensitive toelectrons or to ultraviolet radiations.

According to an embodiment, said layer is defined by a DNA origami.

According to an embodiment:

-   -   the nanowires have a transverse dimension smaller than 300 nm,        preferably smaller than 50 nm;    -   the nanowires have a length greater than 500 nm, preferably        greater than 1 μm; and/or    -   said layer has a thickness smaller than 100 nm, preferably        smaller than 50 nm.

According to a second aspect, an embodiment provides a nanowire formingmethod, comprising the forming of a DNA origami having through openings,and the forming in the through openings of portions forming all or partof the nanowires.

According to an embodiment, said portions are deposited in a chemicalbath.

According to an embodiment, the origami and the openings that itcomprises are formed before the bonding of the origami to a substrate.

According to an embodiment, said portions form first portions of thenanowires, and second portions of the nanowires extending from the firstportions are deposited in a chemical bath.

According to an embodiment, the composition of the chemical bath isdifferent for the forming of the first and second portions.

According to an embodiment, the method comprises the forming of apolymer matrix between the nanowires.

According to an embodiment, the method comprises the removal of at leasta portion of the DNA origami.

An embodiment provided a device obtained by a method such as definedhereabove, wherein the origami comprises folded DNA strands havingportions bound to one another by staples.

According to an embodiment, the DNA strand is located on a layer made ofa same material as that of the nanowires, said portions extending fromsaid layer.

According to an embodiment, the DNA origami is located on a metal regionand, preferably:

-   -   said metal region has a thickness greater than 100 nm; and/or    -   said portions extend from said metal region.

According to an embodiment, the device comprises, at an end of thenanowires opposite to said metal region, an electrically-conductiveregion in contact with the nanowires.

According to an embodiment:

-   -   the metal region comprises at least one of the materials from        the group formed of gold, nickel, copper, palladium, and        platinum; and/or    -   the nanowires are piezoelectric, preferably, the nanowires        having a wurtzite-type crystal structure and/or comprise at        least one of the materials from the group formed of zinc oxide,        cadmium sulfide, cadmium selenide, and nickel selenide.

An embodiment provides a device wherein:

-   -   the nanowires have a transverse dimension smaller than 40 nm,        preferably smaller than 20 nm;    -   the nanowires have a length greater than 500 nm, preferably        greater than 1 μm; and/or    -   the nanowires have a density greater than 10 nanowires per        square micrometer, preferably greater than 50 nanowires per        square micrometer; and/or    -   the DNA origami has a thickness in the range from in the order        of 2 nm to in the order of 100 nm, preferably in the order of 10        nm.

An embodiment provides a sensor pixel, comprising a device such asdefined hereabove.

An embodiment provides a sensor, preferably of fingerprints, comprisinga plurality of pixels such as defined hereabove.

According to an embodiment, the pixels are located on the side of asurface of a substrate comprising, vertically in line with each pixel,at least a portion of a circuit associated with this pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features and advantages, as well as others, will bedescribed in detail in the following description of specific embodimentsgiven by way of illustration and not limitation with reference to theaccompanying drawings, in which:

FIG. 1 is a partial simplified cross-section view showing a step of afirst embodiment of a nanowire manufacturing method;

FIG. 2 is a partial simplified cross-section view showing another stepof the first embodiment;

FIG. 3 is a partial simplified cross-section view showing another stepof the first embodiment;

FIG. 4 is a partial simplified cross-section view showing a step of asecond embodiment of a nanowire manufacturing method;

FIG. 5 is a partial simplified cross-section view showing another stepof the second embodiment;

FIG. 6 is a partial simplified cross-section view showing another stepof the second embodiment;

FIG. 7 is a partial simplified cross-section view showing a step of anexample of a method of manufacturing a sensor comprising nanowires,implementing the embodiments of FIGS. 1 to 6 ;

FIG. 8 is a partial simplified cross-section view showing another stepof the method example; and

FIG. 9 is a partial simplified cross-section view showing another stepof the method example.

DESCRIPTION OF THE EMBODIMENTS

Like features have been designated by like references in the variousfigures. In particular, the structural and/or functional features thatare common among the various embodiments may have the same referencesand may dispose identical structural, dimensional and materialproperties.

For the sake of clarity, only the steps and elements that are useful foran understanding of the embodiments described herein have beenillustrated and described in detail. In particular, steps of formingand/or of removal of various portions of structures are not described indetail, the described embodiments being compatible with usual steps offorming/removal of such portions.

Unless specified otherwise, when reference is made to two elementsconnected together, this signifies a direct connection without anyintermediate elements other than conductors, and when reference is madeto two elements coupled together, this signifies that these two elementscan be connected or they can be coupled via one or more other elements.

In the following description, when reference is made to terms qualifyingabsolute positions, such as terms “top”, “bottom”, “left”, “right”,etc., or relative positions, such as terms “above”, “under”, “upper”,“lower”, etc., or to terms qualifying directions, such as terms“horizontal”, “vertical”, etc., unless otherwise indicated, it isreferred to the orientation of the drawings.

Unless specified otherwise, the expressions “around”, “approximately”,“substantially” and “in the order of” signify within 10%, and preferablywithin 5%.

FIGS. 1 to 3 are partial simplified cross-section views showingsuccessive steps of a first implementation mode of a nanowiremanufacturing method. More precisely, the nanowires are formed bychemical bath deposition. During such a deposition, a seed surface isplaced in contact with a solution defining the chemical bath. Thenanowires grow on the seed surface. The material of the nanowires formsfrom the dissolved content of the solution.

At the step of FIG. 1 , a support 110 is provided. As an example,support 110 is a semiconductor wafer, preferably made of silicon.Preferably, the semiconductor wafer has a front surface (upper surfacein the drawings) covered with an insulating layer, not shown.

Support 110 is covered with a metal layer 120. The metal layer thusdefines a metal region. As a variant, support 110 is metallic anddefines the metal region. The upper surface of metal layer 120, that is,the free surface of the metal region, is intended to form a seed surfaceon which the future nanowires will be formed in the rest of the method.The thickness of metal layer 120 is preferably greater than or equal to40 nm or approximately 40 nm, for example, greater than 50 nm, morepreferably greater than 100 nm, more preferably still greater than 150nm. As compared with a thinner layer, this enables to improve the stateof the seed surface of layer 120.

Metal region 120 may be made of any metal. However, preferably, the meshparameter of the metal and the crystal orientation of metal region 120are adapted to the forming of the crystal lattice of the nanowires. Forthis purpose, metal region 120 is preferably from the group formed ofgold, nickel, copper, palladium, and platinum. More preferably, themetal region is made of gold.

Preferably, before forming metal layer 120, support 110 is covered witha bonding layer, not shown, for example made of chromium or of titanium.The bonding layer enables, as compared with an embodiment where thislayer is omitted, to ease the forming of metal layer 120 and avoidvarious problems of stability and/or of adherence of metal layer 120 onsupport 110.

One then forms, on the free surface of metal region 120, a layer 130.Layer 130 is a perforated layer, that is, it has, or exhibits, throughopenings 135.

Openings 135 are preferably arranged in an array, more preferably in aregular array such as an array having, in top view (that is, seen fromthe upper portion of FIG. 1 ), a symmetry of order two, three, four, orsix. Openings 135 are preferably arranged in an array, the array havingthe same row and column pitches.

Each opening 135 has for example a cross-section shape, that is, a shapein top view, which is rectangular or, preferably, square. Thecross-section may also have a rounded shape, for example, circular.Preferably, all openings 135 have the same cross-section shape and, morepreferably, the same cross-section dimensions.

Preferably, openings 135 have a transverse dimension A smaller than 300nm, more preferably smaller than 50 nm, more preferably still smallerthan 20 nm. Transverse dimension A is further preferably greater than 5nm, more preferably greater than approximately 10 nm. Preferably, thearray of openings 135 exhibits a distance B between neighboring openingsin the range from 0.5 to 3 times the transverse dimension A of openings135. By distance between two openings, there is meant the distanceseparating the closest edges of the two openings. In the case of aregular pattern, the pitch of the pattern is defined by value A+B.

Perforated layer 130 may be obtained by electronic lithography from anelectron-sensitive layer, for example, a layer of poly(methylmethacrylate), PMMA. Perforated layer 130 may also be obtained bylithography with ultraviolet rays, preferably with so-called deepultraviolet rays, that is, having wavelengths shorter than 200 nm. Theperforated layer then results from a layer sensitive to ultravioletrays.

Perforated layer 130 may also be obtained from a layer comprising ablock copolymer. A block copolymer is defined by an association of atleast two immiscible and chemically bonded polymers. Each of thepolymers defines a block of the copolymer. The immiscible characterresults in the forming of separate phases, and one of the phases is thenremoved to form openings 135. The size of the blocks is then selected toobtain the desired dimensions of openings 135.

Perforated layer 130 may also be replaced with an origami ofdeoxyribonucleic acid (DNA) such as that of the embodiments describedhereafter in relation with FIGS. 4 to 6 .

At the step of FIG. 2 , the material of the nanowires is deposited by achemical bath. Portions 210 of the deposited material are formed inopenings 135 on metal region 120. Portions 210 extend from metal region120. More particularly, portions 210 are in contact with metal region120. The deposited material may be any material capable of being formedby chemical bath deposition. Pawar et al.'s publication, entitled“Recent status of chemical bath deposited metal chalcogenide and metaloxide thin films” in Current Applied Physics 11 (2011) 117-161,describes such materials and compositions of chemical baths enabling todeposit these materials. The deposited material may be a metal oxidesuch as nickel oxide (NiO), silver oxide (AgO), copper oxide (Cu₂O), orfor example cadmium oxide (CdO). The deposited material may be a metalhydroxide, preferably iron hydroxide (III) (FeOOH), or copper hydroxide(Cu(OH)₂). The deposited material may also be a chalcogenide, such ascadmium sulfide (CdS), zinc sulfide (ZnS), lead sulfide (PbS), cadmiumselenide (CdSe), zinc selenide (ZnSe), or for example, nickel selenide(NiSe). Preferably, the deposited material has piezoelectric properties,more preferably, the deposited material is zinc oxide (ZnO).

Preferably, the temperature of the chemical bath is in the range from60° C. to 100° C. The deposition is preferably performed for a durationin the range from 1 minute to 60 minutes, more preferably from 5 minutesto 30 minutes.

Although the shown structure has its layer 130 facing the top of thedrawing, the deposition is preferably performed with layer 130 facingdownwards. This enables to protect portions 210 against possibleparticles of the material that might precipitate in the chemical bath.The deposition is preferably performed in the absence of an electricfield in the solution, which enables to simplify the deposition method.

According to an embodiment, each portion 210 forms a nanowire.Preferably, portions 210 entirely fill openings 135, and the length ofthe nanowires is then equal to the thickness of perforated layer 130.According to another embodiment, described hereafter in relation withFIG. 3 , each nanowire is formed by chemical bath deposition on one ofportions 210.

Thus, the number and the positions of the nanowires correspond to thenumber and to the positions of portions 210. Now, perforated layer 130enables to obtain a number of portions 210 greater than the number ofportions which would be obtained by omitting perforated layer 130. Theperforated layer thus enables to increase the number of nanowires persurface area unit.

Further, due to the fact that the positions of the obtained portions 210correspond to those of openings 135, perforated layer 130 enables toobtain portions 210 more regularly distributed than portions which wouldbe obtained without perforated layer 130. Perforated layer 130 thusenables to increase the regularity of the positions of the nanowires.

The transverse dimensions of the nanowires are the transverse dimensionsof portions 210 (that is, lateral dimensions in the orientation of thedrawing), or are a function of the transverse or lateral dimensions ofportions 210. Further, each portion 210 has lateral dimensions smallerthan, preferably equal to, the transverse dimensions of openings 135.Thus, perforated layer 130 enables to obtain the desired dimensions ofthe nanowires more easily than in the absence of a perforated layer.

In the preferred case of forming of ZnO nanowires on a gold metal layer120, the chemical bath for forming portions 210 preferably comprises, insolution:

-   -   zinc nitrate, Zn(NO₃)₂, and Hexamethylenetetramine, HMTA, in        concentrations smaller than 10 mmol/L; and/or    -   Zn(NO₃)₂ in a superstoichiometric concentration with respect to        an HMTA concentration; and/or    -   one or a plurality of additives adapted to favoring a transverse        growth (parallel to layer 130) of portions 210, over a growth of        portions 210 in the thickness direction of layer 130. This or        these additive(s) preferably comprise citrate or chloride ions.

The inventors have observed that the above-defined compositions of thechemical bath, particularly a concentration smaller than the thresholdof 10 mmol/L (unit often noted mM) of Zn(NO₃)₂ and of HTMA, enable toguarantee that a portion 210 is formed in each of openings 135. Thesecompositions further enable to ascertain that, in each opening 135,portion 210 entirely covers the bottom of opening 135. In other words,the number of portions 210 is equal to that of openings 135 and thetransverse dimensions of each portion 210 may be equal to those ofopenings 135. The nanowires resulting from such a chemical bath are thusmore regularly distributed and/or have more regular transversedimensions than for chemical baths having different compositions.

Based on the compositions defined hereabove to form ZnO nanowires on agold metal region 120, those skilled in the art are capable of defining,by routine tests, Zn(NO₃)₂ and HMTA concentration thresholds for othermetals of the metal region.

Those skilled in the art are further capable of defining chemical bathcompositions adapted to the deposition of other materials than ZnO, suchas those defined hereabove, or the materials of the nanowires of theexample of a sensor described hereafter in relation with FIGS. 7 to 9 .In particular, Zn(NO₃)₂ and HMTA form, in the case of the forming of ZnOnanowires, first and second respective compounds capable of being, inthe case of the forming of other materials, different from Zn(NO₃)₂and/or HMTA.

The first compound has, when it is in solution in the chemical bath,cations of at least one metal used to form the formed nanowires. Inother words, the first compound forms a source of metal cations. This orthese metal(s) are preferably from the group formed of zinc (Zn),cadmium (Cd), nickel (Ni), silver (Ag), and copper (Cu). As an example,the first compound comprises one or a plurality of components amongnitrate, acetate, chloride, or for example, sulfate, of the consideredmetal(s). The first compound may thus comprise or be formed of one or aplurality of components among zinc nitrate (Zn(NO₃)₂), zinc acetate(Zn(CH₃COO)₂), zinc chloride (ZnCl₂), zinc sulfate (ZnSO₄), and moregenerally components in form M(NO₃)₂, MNO₃, M(CH₃COO)₂, M(CH₃COO), MCl₂,MCl, MSO₄, where M is a metal preferably from the above-described list.

In the case where the formed material is a metal oxide, the secondcompound comprises a source of hydroxide ions, OH⁻. As an example, thesecond compound may comprise, preferably be made of, an amine, moreparticularly hexamethylenetetramine (HTMA) and/or ammoniac (NH₃), and/orsodium hydroxide (NaOH). The second compound may thus enable to adjustthe pH of the solution to a value adapted to the deposition. In the casewhere the formed material is a metal sulfide, the second compoundpreferably comprises a sulfide source, for example, comprises thiourea(CS(NH₂)₂) or sodium sulfide (Na₂S). In the case where the formedmaterial is a metal selenide, the second compound preferably comprises aselenide source, for example, comprises selenourea (CSe(NH₂)₂), orsodium selenite (Na₂SeSO₃).

Those skilled in the art are then capable of defining, by routine tests,a concentration threshold so that, when the concentrations of the firstand second compounds are lower than this threshold, the transversegrowth of portions 210 is favored, for example, over a growth ofportions 210 in the thickness direction of layer 130. This threshold mayhave a value in the order of 10 mM, for example, equal to 10 mM. It ishere considered that the concentrations of the first and secondcompounds correspond to their concentrations at the time of theirintroduction into the growth bath. The provision of concentrations lowerthan the above-defined concentration threshold enables to form a portion210 in each of openings 135.

The step of FIG. 3 is implemented when, at the end of the step of FIG. 2, each portion 210 only forms a first portion of a future nanowire.Second portions 310 of the nanowires are then deposited in a chemicalbath. Portions 310 extend from portions 210 away from layer 130. Theassembly of a portion 210 and of portion 310, formed on portion 210,forms a nanowire 320.

Preferably, the temperature of the chemical bath is in the range from60° C. to 100° C. The deposition is preferably performed for a durationin the range from 1 minute to 180 minutes according to the length of thenanowires which is desired to be obtained. Nanowires 320 have a lengthgreater than 500 nm, preferably greater than 1 μm.

The step of FIG. 3 thus enables to obtain nanowires 320 having a lengthgreater than the thickness of perforated layer 130. Preferably,perforated layer 130 then has a thickness smaller than 100 nm,preferably smaller than 50 nm. As compared with a thicker perforatedlayer 130, this enables to accelerate the step of FIG. 2 and enables toincrease the free length of nanowires 320. By free length, there ismeant a length over which nanowires 320 are not surrounded with a solidmaterial. The larger the free length, the more easily deformable thenanowires, which advantageously increases the sensitivity of a sensorusing nanowire deformations.

An advantage of the step of FIG. 3 is that nanowires 320 may have a formfactor, defined by the ratio of the length of the nanowires to thesmallest transverse dimensions of the nanowires, greater than the formfactor of nanowires formed of portions 210 only.

The composition of the chemical bath used to form portions 310 isdifferent from that of the chemical bath used to form portions 210.Thus, in the example of forming of ZnO nanowires, the chemical bathcomprises, in solution: Zn(NO₃)₂ and HMTA, in concentrations greaterthan 20 mM; and/or

-   -   Zn(NO₃)₂ in a substoichiometric concentration with respect to an        HTMA concentration; and/or    -   one or a plurality of additives adapted to favoring a growth of        portions 310 orthogonally to layer 130 over a transverse growth        of portions 310. Such an additive may comprise a polyethylene        imine PEI, or ethylene diamine.

The above-described composition of the chemical bath enables to form,from portions 210, portions 310 extending vertically, that is,orthogonally to the surface of metal region 120, in other wordsorthogonally to layer 130. Portions 310 having transverse dimensionssubstantially equal to the transverse dimensions of portions 210 canthus be obtained. This enables to obtain nanowires having asubstantially constant cross-section along substantially the entirelength of each nanowire. In other words, the nanowires are substantiallycylindrical, with an axis of revolution or not, or substantiallyprismatic. As compared with nanowires having non-constantcross-sections, nanowires with a constant cross-section enable toimprove the operation of a device using these nanowires with a constantcross-section.

In the same way as for the chemical bath described in relation with FIG.2 , the chemical bath described in relation with FIG. 3 may be adaptedto other deposited materials than ZnO. For this purpose, Zn(NO₃)₂ andHMTA respectively form the first and second above-described compounds,which may be different from Zn(NO₃)₂ and HMTA. In particular, thoseskilled in the art are capable of determining, by routine tests:

a concentration threshold so that, when the concentrations of the firstand second compounds are lower than this threshold, the transversegrowth of portions 310 orthogonally to layer 130 is favored over atransverse growth of portions 310. This threshold may have a value inthe order of 20 mM, for example, equal to 20 mM; and/or additives alsoenabling to favor the growth of portions 310 orthogonally to layer 130over a transverse growth of portions 310.

FIGS. 4 to 6 are partial simplified cross-section views showingsuccessive steps of a second embodiment of a nanowire manufacturingmethod.

At the step of FIG. 4 , a support 110 and a metal region 120, identicalor similar to those described in relation with FIG. 1 and arrangedidentically or similarly, are provided.

According to the shown embodiment, a seed layer 410 is formed on metalregion 120. The future nanowires will be formed on top of and in contactwith the free surface of seed layer 410. Preferably, seed layer 410comprises, for example, is made of, the same material as that of thefuture nanowires. Preferably, seed layer 410 is made of ZnO. Morepreferably, the seed layer is a layer of ZnO nanoparticles. Bynanoparticles, there is meant particles having their largest dimensionssmaller than one micrometer, preferably smaller than 500 nm. In avariant (not shown), metal region 120 is omitted. In another variant(not shown), layer 410 and metal region 120 are omitted and the uppersurface of support 110 defines a seed surface.

According to another embodiment, layer 410 is omitted and the surface ofmetal region 120 forms a seed surface, as described in relation withFIGS. 1 to 3 .

There is formed on the seed surface an origami of deoxyribonucleic acidDNA 430. By DNA origami, there is meant a three-dimensional structureformed by an assembly of folded DNA strands. Portions of the differentDNA strands are bound to one another by staples. The staples arepreferably pieces of DNA. The bases of DNA strands are selected, forexample, by means of a current software, so that the folding of the DNAstrands in aqueous solution in the presence of the staples forms thedesired three-dimensional structure.

DNA origami 430 is in contact with the seed surface and covers all orpart of the seed surface. Thus, DNA origami 430 is preferably located onlayers 120 and 410. In the case where layer 410 is omitted and metallayer 420 is made of gold and defines the seed layer, thiol groups maybe provided to bind the DNA origami 430 to the seed surface.

The DNA origami 430 exhibits through openings 435. DNA origami 430 hasan average thickness C, defined outside of the openings. Averagethickness C is preferably uniform over the seed surface or over theportion of the seed surface covered with the DNA origami. Thus, the DNAorigami has the three-dimensional structure of a perforated layer.Preferably, average thickness C is in the range from a few nanometers,that is, from in the order of 2 nm to 10 nm, to a few tens ofnanometers, that is, from in the order of 20 nm to 100 nm. Morepreferably, average thickness C is in the order of 10 nm, for example,equal to 10 nm.

Openings 435 are preferable arranged in an array, more preferably in aregular array such as an array having, in top view, a symmetry of ordertwo, three, four, or six. Openings 435 are preferably arranged in anarray, the array having the same row and column pitches. As an example,the row and column pitch is in the range from 15 nm to 30 nm, preferablyequal to 20 nm or to 25 nm.

Each opening 435 for example has a rectangular or, preferably, square,cross-section shape. The cross-section of each opening 435 may also havea rounded shape, for example, substantially circular. Preferably, allopenings 435 have the same cross-section shape and the samecross-section dimensions.

Openings 435 for example have a transverse dimension A1 smaller than 100nm, preferably smaller than 40 nm, more preferably smaller than 20 nm,more preferably still smaller than 15 nm. Transverse dimension A1 isfurther preferably greater than 5 nm. More preferably, transversedimension A1 is in the order of 10 nm. Preferably, the array of openings435 has a distance B1 between neighboring openings in the range from 0.5to 3 times the transverse dimensions A1 of openings 435.

At the step of FIG. 5 , openings 435 are filled with the material of thenanowires. Portions 210 of the material are thus formed in openings 435.Portions 210 extend from the seed surface. More precisely, portions 210are in contact with seed layer 410 or, if the latter is omitted, withmetal layer 120. The material of portions 210 may be any materialcapable of being formed by chemical bath deposition, such as describedin relation with FIG. 2 . Thus, the material of portions 210 may be ametal oxide such as NiO, AgO, Cu₂O, or for example, CdO. The depositionmaterial may be a metal hydroxide, preferably FeOOH or Cu(OH)₂. Thedeposited material may also be a chalcogenide, CdS, ZnS, PbS, CdSe,ZnSe, or for example NiSe. Preferably, the deposited material haspiezoelectric properties, more preferably, the material of portions 210is zinc oxide (ZnO).

Preferably, portions 210 are formed by chemical bath deposition, asdescribed in relation with FIG. 2 . This is not limiting, and portions210 may be formed by any usual method enabling to form portions inthrough openings of a perforated layer. As an example, the depositionmay be performed in the presence or in the absence of an electric field,or by electrolysis. The deposition may also be performed over the entiresurface of the structure obtained at the step of FIG. 4 , the portionslocated above the upper level of DNA origami 430 being then possiblyremoved, partly or totally, for example, by polishing.

Preferably, at the step of FIG. 6 , the DNA origami (430, FIG. 5 ) isremoved. This removal is for example performed by a plasma adapted toselectively etching the DNA origami over the material of portions 210.

According to an embodiment, each portion 210 forms a nanowire.Preferably, portions 210 entirely fill openings (435, FIG. 5 ), and thelength of the nanowires is then equal to thickness C (FIG. 4 ) of theDNA origami (430, FIG. 5 ).

According to another embodiment, each nanowire is formed by chemicalbath deposition on one of portions 210, as described hereabove inrelation with FIG. 3 . The nanowires then preferably have a lengthgreater than 500 nm, more preferably greater than 1 μm. The step of FIG.6 may then be omitted.

It could as a variant be devised to replace the DNA origami with aperforated layer such as the layer 130 of the method of FIGS. 1 to 3 ,this perforated layer being obtained by electron lithography, byultraviolet radiation, or from a block copolymer. However, as comparedwith such as variant, the DNA origami enables to obtain nanowires ofsmaller diameter and/or to reach a greater density of nanowires on thesurface. As an example, the diameter of the nanowires may be smallerthan 10 nm. Further, the nanowire density is preferably greater than 10nanowires per μm², for example, greater than 50 nanowires per μm²,preferably equal to, or greater than, 625 per μm², or even equal to, orgreater than, 1,600 per μm². The DNA origami thus enables to improve theaccuracy and the sensitivity of a sensor using the obtained nanowires.

FIGS. 7 to 9 are partial simplified cross-section views, showing stepsof an example of a method of manufacturing a sensor comprisingnanowires. This method implements the steps of FIGS. 1 to 3 or the stepsof FIGS. 4 to 6 . The sensor is for example a fingerprint sensor. Inparticular, the sensor comprises a pixel array. A single pixel has beenshown, the other pixels being similar or identical to the shown pixel.

At the step of FIG. 7 , a support 110 formed by a semiconductorsubstrate, for example, made of silicon, is provided. Preferably, foreach pixel, the sensor comprises a pixel control/read circuit, notshown, comprising transistors, for example, MOS-type transistors. Thecircuit is preferably of CMOS type.

Preferably, all or part of the transistors of the circuit associatedwith each pixel are located inside and on top of substrate 110,vertically in line with a location 710 where the nanowires will beformed. The location 710 of each pixel typically has a square shape intop view. For example, the dimensions of sides of location 710 are inthe range from 0.8 μm to 1.5 μm, preferably are of approximately 1 μm,more preferably are equal to 1 μm.

Each pixel comprises two electrically-conductive regions 720 and 722located on the front surface side of substrate 110 (that is, in theupper portion of the substrate). Electrically-conductive regions 722 areelectrically connected to the circuit associated with the pixel.

The front surface of substrate 110 is covered with anelectrically-insulating layer 730, typically made of silicon oxide.Insulating layer 730 is thoroughly crossed by a conductive via 732located on conductive region 722.

A metal layer 740 covering insulating layer 730 is then formed. Metallayer 740 forms a metal conductive region. Conductive via 732 placesmetal layer 740 in electric contact with conductive region 722. Anopening 742 is provided in metal layer 740 vertically in line with, thatis, in front of, conductive region 720. Metal layer 740 is identical orsimilar to the metal layer 120 of the embodiments described in relationwith FIGS. 1 to 6 .

One forms, on metal layer 740, nanowires 750 as described hereabove inrelation with FIGS. 1 to 3 and/or with FIGS. 4 to 6 to form nanowires210 or 320. Preferably, nanowires 750 are in contact with metal layer740.

Preferably, nanowires 750 are formed only at location 710. For thispurpose, in the case where nanowires 750 are formed in an aqueoussolution such as a chemical bath, the surface of metal layer 740 may bemade hydrophobic outside of location 710.

At the step of FIG. 8 , a polymer layer 810 filling the space betweennanowires 750 is formed. Layer 810 thus forms an electrically-insulatingpolymer matrix. More precisely, the polymer is more flexible than thematerial of nanowires 750, that is, it has a modulus of elasticitysmaller, for example, more than 10 times smaller, that that of nanowires750. Preferably, layer 810 is formed so that the upper end of nanowires750, that is, the end of nanowires 750 opposite to metal layer 740, isflush with the upper surface of layer 810. Preferably, layer 810 coversthe front surface of the structure obtained at the step of FIG. 7outside of location 710. Layer 810 is in contact with insulating layer730 in the opening 742 of metal layer 740.

At the step of FIG. 9 , a conductive via 910 thoroughly crossing layer810 and located vertically in line with conductive region 720 is formed.Conductive via 910 runs through the opening 742 of conductive layer 740.Conductive via 910 is insulated from the lateral walls of opening 742 byportions of layer 810.

An electrically-conductive region 920 covering nanowires 750 is alsoformed. Conductive region 920 is in contact with the upper ends ofnanowires 750. Conductive region 920 may be made of the same material asvia 910. Regions 920 and the material of via 910 may then besimultaneously formed. Conductive region 920 may also be formed afterthe material of via 910, and conductive region 920 and conductive via910 may then be made of different materials. In a variant, the materialof conductive layer 920 is transparent in a wavelength range. As anexample, the wavelength range corresponds to visible radiations, thatis, in the range from approximately 400 nm to approximately 800 nm. Bytransparent layer, there is meant that more than 50%, preferably morethan 90%, of any radiation in the wavelength range entering the layerperpendicularly through one of the main surfaces of the layer (parallelto the plane of the layer) comes out of the layer through the other oneof the main surfaces. In this variant, layer 810 is also transparent.When the sensor is submitted to a radiation crossing conductive layer920, this radiation can thus be detected due to its interaction with thenanowires.

Preferably, the conductive regions 920 of neighboring pixels areinsulated from one another. To achieve this, conductive regions 920 arepreferably obtained by steps of: forming a conductive layer comprisingthe future conductive regions 920;

-   -   covering this conductive layer with a lithographed layer, not        shown, exhibiting openings outside of the locations of        conductive regions 920; and    -   etching the portions of the conductive layer located vertically        in line with the openings of the lithographed layer.

In each pixel thus obtained, nanowires 750 are arranged parallel to oneanother and have their end in respective contact with conductive regions740 and 920. Conductive regions 740 and 920 are in electric contact withrespective regions 722 and 720 via respective vias 732 and 910. Regions722 and 720 form the electrodes of the pixel.

The fact for substrate 110 to comprise, vertically in line with eachpixel, at least a portion of the circuit associated with this pixel,enables, with respect to a sensor where the circuits are not verticallyin line with the pixels, to increase the compactness and/or theresolution of the sensor. Further, the fact for the nanowires to havetransverse dimensions such as defined hereabove enables, as comparedwith nanowires having larger lateral dimensions, to decrease the pixelsize and thus increase the sensor resolution. Thus, the resolution ofthe obtained sensor may be smaller than 50 μm, preferably in the orderof 1 μm.

Preferably, the nanowires are piezoelectric, that is, are made of apiezoelectric material. In operation, a pressure or a force exerted onregion 920, for example, towards the bottom of FIG. 9 , deforms thenanowires and causes a potential difference measured by the circuitassociated with the pixel. For this purpose, as an example, the materialof the nanowires is selected from among zinc oxide, ZnO, cadmiumsulfide, CdS, and cadmium selenide, CdSe. The piezoelectric material mayalso be selected from among materials having a wurtzite-type crystalstructure. The nanowires may also comprise a plurality of thesematerials.

It is preferred for metal region 740 to be directly in contact withnanowires 750. A Schottky-type electric contact is thus preferablyformed between metal region 740 and nanowires 750. The sensitivity ofthe sensor is then advantageously greater than that of a similar sensorbut further comprising a layer such as layer 410 (FIGS. 4 to 6 ) locatedbetween metal region 740 and nanowires 750.

Various embodiments and variants have been described. Those skilled inthe art will understand that certain features of these embodiments canbe combined and other variants will readily occur to those skilled inthe art.

In particular, although embodiments where the forming of origami 430 onlayer 120 or 740 is performed while this layer is supported by support110 has been more particularly been described, it may be provided,according to another embodiment, to form origami 430 on layer 120 beforetransferring it onto support or substrate 110.

Finally, the practical implementation of the described embodiments andvariations is within the abilities of those skilled in the art based onthe functional indications given hereabove.

1. Method of forming nanowires, comprising the forming of a DNA origami having through openings, and the forming in the through openings of portions forming all or part of the nanowires.
 2. Method according to claim 1, wherein the origami and the openings that it comprises are formed before the bonding of the origami onto a substrate.
 3. Method according to claim 1, wherein said portions are deposited in a chemical bath.
 4. Method according to any of claim 1, wherein said portions form first portions of the nanowires, and second portions of the nanowires extending from the first portions are deposited in a chemical bath.
 5. Method according to claim 4, wherein the composition of the chemical bath is different for the forming of the first and second portions.
 6. Method according to claim 1, comprising the forming of a polymer matrix between the nanowires.
 7. Method according to claim 1, comprising the removal of at least a portion of the DNA.
 8. Device obtained by a method according to claim 1, wherein the origami comprises folded DNA strands having portions bound to one another by staples.
 9. Device according to claim 8, wherein the DNA origami is located on a layer made of a same material as that of the nanowires, said portions extending from said layer.
 10. Device according to claim 8, wherein the DNA origami is located on a metal region and, preferably: said metal region has a thickness greater than 100 nm; and/or said portions extend from said metal region.
 11. Device according to claim 10, comprising, at one end of the nanowires opposite to said metal region, an electrically-conductive region in contact with the nanowires.
 12. Device according to claim 10, wherein: the metal region comprises at least one of the materials from the group formed of gold, nickel, copper, palladium, or platinum; and/or the nanowires are piezoelectric, preferably, the nanowires having a wurtzite-type crystal structure and/or comprise at least one of the materials from the group formed of zinc oxide, cadmium sulfide, cadmium selenide, and nickel selenide.
 13. Device according to claim 8, wherein: the nanowires have a transverse dimension smaller than 40 nm, preferably smaller than 20 nm; the nanowires have a length greater than 500 nm, preferably greater than 1 μm; and/or the nanowires have a density greater than 10 nanowires per square micrometer, preferably greater than 50 nanowires per square micrometer; and/or the DNA origami has a thickness in the range from in the order of 2 nm to in the order of 100 nm, preferably in the order of 10 nm.
 14. Sensor pixel, comprising a device according to claim
 8. 15. Sensor, preferably of fingerprints, comprising a plurality of pixels according to claim
 14. 16. Sensor according to claim 15, wherein the pixels are located on the side of a surface of the substrate comprising, vertically in line with each pixel, at least a portion of a circuit associated with this pixel.
 17. Method according to claim 2, wherein said portions are deposited in a chemical bath. 